Chapter 8

Predicting What Comes Next: Exploring Sequences and Progressions

CBSE Class 9

Mathematics Solutions

Question:

Find the $10^{\text{th}}$ and $15^{\text{th}}$ terms of the sequence $t_n = 5n - 3$ for $n \geq 1$ . [Page No. 179]

Answer: Verified

Direct substitution.

$t_{10} = 5(10) - 3 = 47$

$t_{15} = 5(15) - 3 = 72$

Hence, $t_{10} = 47, t_{15} = 72$ .

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(All Q's of this Chapter solved)

More NCERT Questions

Can the same approach (refer to NCERT page no. 183) be used to find the sum of 1 + 2 + 3 + + 100? [Page No. 184] Observe the Sierpiński triangle. (A) How many black triangles are there in Stages 0 to 3? (B) Predict the number at Stages 4 and 5. (C) Find a rule for the number of black triangles at the nth stage. Can you use this formula to find S20, S50 or S1000? [Page No. 185] Let us revisit the sequence tn of triangular numbers 1, 3, 6, 10, 15, shown in the given figure. Note that the nth term of this sequence is the sum of the first n natural numbers. Thus, tn = n(n + 1)/